1. Field of the Invention
The present invention relates generally to radio communications and more specifically to communication technologies for multiple access in difficult and hostile environments combined with dynamic environment changes.
2. Description of the Prior Art
The communication technology developed in the 1940's during World War II included "frequency diversity communication" or "stacked carrier communications" to aid high frequency (HF) band traffic. J. Proakis refers to frequency diversity communication technology in, Digital Communications, McGraw-Hill, 1989, see, sections 7.4 to 7.7. Diversity techniques are said by Proakis to be based on the notion that errors occur in the reception of largely attenuated channels, e.g., channels in deep fade. Supplying the receiver with several duplicates of the original signal, but over channels that fade independent from one another, has the potential of securing continuous communication except during the unlikely event that all the duplicate channels fade out together. Such probability can be estimated.
Frequency diversity is one of many diversity methods. The same modulation is carried by several carrier channels separated by nominally the coherence bandwidth of each respective channel. In time diversity, the same information is transmitted over different time slots.
Multiple antennas can be used in a diversity scheme. Several receiving antennas can be used to receive the signals sent from a single transmitting antenna. For best effect, the receiving antennas are spaced enough apart to vary different multipath interference amongst the group. A separation of nominally ten wavelengths is generally needed to observe independent signal fading.
A signal having a bandwidth much greater than the coherence bandwidth of the channel can be used in a more sophisticated diversity scheme. Such a signal with a bandwidth W will resolve the multipath components and provide the receiver with several independently fading signal paths.
Other prior art diversity schemes have included angle-of-arrival or spatial diversity and polarization diversity.
When a bandwidth W much greater than the coherence bandwidth of each respective channel is available to a user, the channel can be subdivided into a number of frequency division multiplexed sub-channels having a mutual separation in center frequencies of at least the coherence bandwidth of each respective channel. The same signal can then be transmitted over the frequency-division multiplex sub-channels to establish frequency diversity operation. The same result can be achieved by using a wideband binary signal that covers the bandwidth W.
G. K. Kaleh describes such in an article, "Frequency-Diversity Spread-Spectrum Communication System to Counter Band-limited Gaussian Interference", IEEE Transactions on Communications, September 1994. Here a secure setup is outlined that can operate in deliberately hostile signal environments.
J. Proakis describes frequency diversity spread spectrum and multiple access concepts in chapter eight, "Spread Spectrum Signals for Digital Communication", supra. Diversity transmission combined with frequency-hopping spread spectrum is detailed for protection against multipath fading and partial-band jamming.
Retro-directivity was proposed and used as early as 1959 to adapt a multi-element antenna array to provide identical spatial gain patterns during transmission and reception operations. See, R. Monzingo, T. Miller, Introduction to Adaptive Arrays, Wiley Interscience Publications, 1980; L. Van Atta, "Electromagnetic Reflection," U.S. Pat. No. 2,908,002, 1959; and B. Glance, P. Henry, "High Capacity Mobile Radio System," U.S. Pat. No. 4,383,332, May 10, 1983, for a discussion of such techniques. TDD systems provide an effective means for implementing retrodirective antenna arrays, e.g., by minimizing the channel variation between the reception and transmission paths.